No perspective of where that hole and the oddball screw are located on the transom. I can say that nothing the factory did would have that shape of hole, most likely something a PO did to the boat to maybe fish a cable through for a transducer is my best guess..
My thinking is that the top part of the keyhole was a small hole for a fish finder or speedometer hose. Then at some time they needed to enlarge the hole to get a wiring plug through. Since you can't run a hole saw without a fixed center drill, they started the center drill outside of the original hole and made the larger bottom part of the keyhole.
